8 months ago I could set prority of AI lanes by setting number on ID in External state property.Now, on 3.1.5 version I can't do that and I can't change AI lanes priority on prefabs which are made before 3.1.5 version.How I can do that now?

what exactly happens when you say "it doesn't work" ? it doesn't get exported? have you checked priority on re-imported model? Also, if I'm not mistaken, the highest priority value you can assign is 8.

Edited ID numbers are kept, but it doesn't change behavior of AI traffic.I noticed that if I open SCS prefab on which AI lanes (ofcourse starting points) must have different IDs, all IDs are 0 and changing that doesn't change anything in game.

what exactly happens with AI traffic on modded prefab? I've checked the changes log for this year and no relevant changes found. Only light-control (semaphore) changes were made in prefab code; in late 2016 changes were made to prefab border variations and in year 2015 changes for ETS2 v1.9 (significant prefab changes) were made. Nothing got changed in exporter in past few months.

however, properties access and assignment have been changed from compatibility to settings. there might be some problem in incorrect name->bit assignment, as I haven't checked it. you can easily check whether these are right or wrong by loading any intersection prefab and examining "left blinker" and "right blinker" flags. If these are set wrong, then bits assignment is incorrect. I can't make this test as my current version has changes for newest prefab and is not ready to be used to import (code changes in progress).

make a very simple refab with lanes merging for test. two identical ai lines merging into single line (and splitting again), then make a test round loop road with this prefab placed: you will see how traffic goes on this prefab; then try different IDs on curve points approaching connection point and check whether anything changes in game's AI rules or not.

once again: there were no changes in export code in about a year. something might has changed in game logic, but I can't be sure on this at all.

yup, I'm messing with prefabs and AI lanes at the moment. You should go to "settings->general->view->meshes->manipulators" and set Default color to bright red or magenta, so you explicitly see curve points when spline is on manipulators level. I've been missing some barely-seen points too.

points at end of lanes and at joints/splits are of different color because they are shared. pink color is used to show them as "different from the rest of points". using the same color on arbitrary curve points is not recommended.

I made new AI lane on prefab separated from other AI lanes, only connected with border of prefab.When I add prefab to ATS and rotate, end with arrow always has angle 0, so, when I rotate prefab, new AI lane become curved. I checked properties for exsisting and new lanes and I couldn't find difference. How to solve it?

Your Lane_1 and Lane_7 are not connected to borders. You should select both borders and both these lanes; then press '5', hover any of selected and click to switch all four splines (2 borders, 2 lanes) onto points level. Then use selection (quick auto quard selection) and modify/weld tool (hotkey 'W') to weld vertices at connection points. both start and end points on both AI lanes should be connected with respective points of borders. So you will perform rect-selection and press 'W' after selection, repeat four times.

You are probably rotating whole thing including parent/placement/grouping dummy nodes. These should not be moved/rotated. On the attached image "lanes" dummy is selected, but it should not be selected (like "prefab" dummy). You can select multiple entries in hierarchy view and Alt+click on checkmark next to name to select/deselect them.